In high-speed transmissions and gear assemblies using helical gears, i.e. with peripheral speeds at the contact circle of the teeth in excess of about 100 meters/second, and for high power (e.g. 5 to 100 MW) it is common to provide single-helix, wide gears, i.e. gears having an axial length and a diameter in ratio in excess of 0.8. Because of the friction developed in the meshing teeth of such gears, large amounts of heat are generated. Experiments have shown that such heat is not a significant function of the load, but rather is practically exclusively a function of the peripheral speed of the two gears.
Removal of the heat which is thus generated is vital for effective operation of the transmission, e.g. to limit wear and distortion of the teeth and a toothed periphery of the heat.
The lubrication of the transmission thus has the double function of reducing friction in accordance with conventional lubrication concepts, i.e. by interposing a thin film of a low-friction fluid between the contacting surfaces of the meshing teeth, and also the function of dissipating the heat generated by the meshing teeth. To this end it is the conventional practice to provide a so-called tooth spray whereby the liquid lubricant, i.e. oil, is introduced into the meshing region and is displaced by compression in the gaps between the teeth with an axial component.
As a result the temperature along the length of the wide gear has a distribution as represented in FIG. 1 for a conventional single-helix helical gear of the axial length or width B. In FIG. 1 of the drawing, the temperature has been plotted along the ordinate T and the axial length along the abscissa B, the left- and right-hand ends of the curve representing the opposite axial ends of the gear. As will be apparent from this figure, over approximately the left-hand half of the gear, assuming that the displacement of lubricant is to the right as represented by the arrow A, the temperature increases gradually from a temperature T.sub.1 to a temperature T.sub.2. Between the midpoint of the gear and the right-hand axial end thereof, the temperature rises shortly to a peak T.sub.3 and then falls off to a temperature T.sub.4 not significantly higher than the temperature T.sub.2. Naturally, because of the high temperature in the right-hand half of the gear, the teeth are subject to severe distortion. In this respect reference may be made to Mechanism and Machine Theory, 1973, Vol. 8, Pages 293-303, L. Martinaglia, presented at the International Symposium on Gears and Transmissions, San Fransisco, California, Oct. 11-13, 1972.
Since the temperature distribution in operation is asymmetrical, the distortions resulting from temperature differentials tend to be asymmetrical as well and can cause considerable local wear which may render the system inoperative.
To the present, as far as applicants are aware, there has been no way to prevent the aforedescribed temperature distribution for an ordinary single-helix helical gear and hence no practical way in which the thermal deformations can be prevented.
It has been proposed to compensate for the aforedescribed temperature distribution and the deformations associated therewith by modifying the shapes of the teeth along their lengths so that, even with deformation, there is no increase in wear or such tendency to increased wear is reduced. The systems for correcting the teeth along their lengths, however, are disadvantageous in that they require separate machining processes and prohibitibely raise the cost of the gear and a transmission incorporating same.
Of course, such corrections are only effective for a single operating state of the gear or transmission, since only a specific temperature distribution and temperature increase can be compensated thereby. For other operating states, the correction is ineffective and hence the transmission can only be operated with a constant load at a fixed speed.